You need either really small or really thin.
I have a couple of thin 7.4v nunchuck batteries I got from Wat that were for the MP5. I have a couple of really thin triple nunchuck Turnigy I got from X-Force.
The surprise one was a super tiny 11.1v from ihobby that fits inside my MCX Rattler build which is substantially smaller than a normal MCX handguard.
Be prepared to swap out the battery plugs to suit though.